1. Field of the Invention
This invention relates generally to a method of digitally controlling power converters and particularly to a control strategy for reducing output voltage spikes during transients by incorporating intermediate combination modes in place of direct transitions from one mode to another mode. This invention also can improve the efficiency of power converters by replacing higher loss modes with combination modes.
2. Discussion of Related Art
A common power handling problem, especially for portable applications powered by batteries, such as cellular phones, PDAs, digital cameras, wireless modems and DSL modems, is a need to provide a regulated non-inverting output voltage from a variable input voltage source.
Voltage sources, such as batteries, that power these devices generally vary from a fully-charged voltage to a discharged voltage. Often the device will have a voltage requirement that is between the fully-charged voltage and the discharged voltage. Thus the voltage source must be regulated, e.g., stepped-up or stepped-down as required, to provide an output voltage that meets the voltage requirement of the device.
Regulation of variable voltage sources from one voltage level to another voltage level is generally accomplished by means of a DC-DC power converter. Known power converter topologies that can provide a constant voltage output from a variable voltage input include: Single-Ended Primary Inductance Converters (SEPIC); Ćuk converters; isolated buck-boost converters; and cascaded buck and boost converters.
Known methods of controlling these power converter topologies include bucking (i.e., stepping-down the voltage level) the input voltage when it is greater than the voltage requirement of the device, boosting (i.e., stepping-up the voltage level) the input voltage when it is less than the voltage requirement and buck-boosting when the input voltage is nearly equal to the voltage requirement. Each of these changes, from buck to buck-boost, and from buck-boost to boost, are referred to as transients. Each of these transients will inevitably cause a voltage spike/voltage ripple of the output voltage. Voltage spikes can cause the converter to lose efficiency and can damage electrical devices.
FIG. 1 is a representative graph illustrating voltage versus time for an exemplary device with a variable input voltage source, a battery, and a regulated voltage output requirement. As shown, the battery has an input voltage (Vin) that varies from a fully-charged voltage of 6 V (VinH) to a discharged voltage of 3.6 V (VinL). This application needs to provide a steady output voltage of 5V (Vout). Using a positive buck-boost converter with a prior art control technique, the positive buck-boost converter operates in a buck mode for a timeframe ‘TA’, followed by operation in a buck-boost mode for a timeframe ‘TB’, and finally operation in a boost mode for a timeframe ‘TC’.
Each transition from one mode to another mode results in a sudden change in a duty cycle ratio, as well as a change in the switching of switching devices. In the buck mode, when the input voltage (Vin) is nearly equal to the output voltage (Vout), a buck duty cycle,
            D      buck        =                  V        out                    V                  i          ⁢                                          ⁢          n                      ,will approach 1. In the boost mode, when the input voltage (Vin) approaches the output voltage (Vout), a boost duty cycle,
            D      boost        =          1      -                        V                      i            ⁢                                                  ⁢            n                                    V          out                      ,moves toward zero. In the buck-boost mode, when the input voltage (Vin) equal to the output voltage (Vout) a buck-boost duty cycle,
            D              buck        -        boost              =                  V        out                              V                      i            ⁢                                                  ⁢            n                          +                  V          out                      ,becomes 0.5. As the fully-charged voltage (VinH) decreases towards the discharged voltage (VinL), the duty cycle should follow the pattern of 1 for the buck mode to 0.5 for the buck-boost mode and then to 0 in the boost mode. Output voltage spikes are associated with these sudden changes in the duty cycle.